Many metal-oxide-semiconductor field-effect transistors (MOSFET) designed for high voltage applications (with high breakdown voltage) have a vertical structure. Using a vertical structure, it is possible for the transistor to sustain both high blocking voltage and high current. The voltage rating of the transistor is a function of the doping and thickness of the n-epitaxial layer (“n-epi layer”) in the case of NMOS, while the current rating is a function of the channel width (i.e. the wider the channel, the higher the current). In a planar structure, the current and breakdown voltage ratings are both a function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the “silicon estate.” With the vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness (actually the n-epi layer thickness) is proportional to the breakdown voltage. Vertical MOSFETs are usually designed for switching applications. In general, NMOS is used instead of PMOS for many applications due to better performance given the same dimensions (because of higher mobility of electrons than holes).
In a conventional vertical NMOS, an n+ sinker is generally used for a vertical drain current collection. FIG. 1 illustrates a cross-section of a conventional vertical NMOS with a p-substrate. The vertical NMOS has a p-substrate 102, an n+ buried layer (NBL) 104 and an n-sinker 108 for a vertical drain current collection, an n-epi layer 106, p-bodies 110, source n+ regions 112 and p+ regions 114 both connected to the source contact 116, a gate 118, and a drain contact 120. The n-sinker 108 requires a large lateral space 122 for isolation between the source 116 and the drain 120. However, the required space 122 for the isolation will increase the device area and lead to an increased RDSon (drain to source resistance in on-state). In addition, the profile of the n-sinker 108 is hard to control because of different thermal budget. The n-sinker 108 is used as a vertical connection between NBL 104 and drain contact 120. Since a high-energy implant step has its limitation in the implant depth, a larger thermal driver-in is required to push implant atoms deeper. In this kind of thermal, the n-sinker 108 receives a large thermal budget (temperature x hours) and results in an isotropic diffusion. Thus, the profile of n-sinker 108 becomes broader and deeper, which leads to a connection of NBL 104 with n-sinker 108 having an unwanted device area. Further, a multi-implant step is required for a deep n-sinker 108. Therefore, the body of the n-sinker 108 will become broader than expectation and the body of n-sinker 108 occupies an extra device area.
FIG. 2 illustrates a cross-section of another conventional vertical NMOS with a silicon-on-insulator (SOI) wafer. The NMOS has a p-substrate 102, a buried oxide (BOX) layer 202, n+ regions 204 connected to a drain contact 120, an n-epi layer 106, p-wells 210, source n+ regions 112 and p+ regions 114 both connected to the source contact 116, a gate 118, and isolation oxide layers 206 and 208. The oxide layers 206 and 208 provide pn-junction isolation and a higher break down voltage. The BOX layer 202 is also required for high voltage operation. Still, this structure requires a large lateral space 122 for isolation between the source 116 and the drain 120.
Accordingly, new methods and structures to reduce the required device area and to have a high breakdown voltage for high-side operations are needed.